1) A lack of faith in central banks and their long-term control over government currencies
2) A desire to hold currency that has positive expected value when more efficient means of production are discovered.
That is all.
-bgc
That is a really good answer indeed. Bitcoin is also backed by loads of people. I've read a lot of other good answers in this thread and it has made me understand a lot about bitcoin that I really couldn't explain to some one who was unfamiliar with this.